Cast iron has been used in large quantities for many years because of such desirable characteristics as good castability, good machinability, and low cost. In spite of these advantages, it has not been used extensively in elevated-temperature applications because very little is known about its load-carrying ability at these temperatures. Although present ASTM specifications, Designation A278-53, limit the use of gray cast iron to a maximum temperature of 650° F, insufficient data are available to establish the equity and reliability of this temperature limitation. Under the sponsorship of the Joint ASTM-ASME Committee on the Effects of Temperature on the Properties of Metals, therefore, an investigation was conducted of the properties of cast iron in the temperature range of 800° F to 1000° F. Most of the work was carried out on gray cast irons of relatively low alloy content, although one nodular-iron alloy was also thoroughly tested primarily for comparative purposes. The purpose of the investigation was to determine whether low-alloy cast irons can safely be used for load-carrying applications at temperatures above the presently specified maximum of 650° F. This investigation included the following main phases of work: (1) a literature survey of previous work pertinent to the subject, (2) screening tests on twelve cast-iron alloys, (3) creep-rupture tests on seven alloys at 800° F and at 1000° F, (4) thermal-shock tests on the same seven alloys at 800° F, and (5) growth tests on the same seven alloys. To complement these main phases of work, auxiliary tensile tests, metallographic studies, and studies of heat treating effects were carried out. All of the alloys used in this work were furnished in the form of keel-block legs by commercial foundries.